Conventionally, the detection element of a gas sensor used to detect the concentration of a particular gas contained in gas to be detected is formed of ceramic, such as zirconia, and is formed such that a closed-bottomed tubular solid electrolyte body having a closed end intervenes between a pair of electrodes. The pair of the electrodes is formed of, for example, platinum or a platinum alloy. Also, a pair of lead wires is electrically connected to the pair of electrodes for leading out output of the detection element. A heater having, at its forward end portion, a heat-generating resistor for generating heat through energization is disposed within a forward portion of the detection element for activating the solid electrolyte body through application of heat.
Such a detection element detects a particular gas component (e.g., oxygen) contained in gas taken in or exhausted from an internal combustion engine of an automobile or the like. For example, in an oxygen sensor for detecting oxygen contained in exhaust gas from an internal combustion engine of an automobile or the like, a pair of electrodes consists of a measuring electrode exposed to exhaust gas and a reference electrode exposed to reference gas (usually, the air), and the pair of electrodes is formed on the surface of the closed-bottomed tubular solid electrolyte body such that the solid electrolyte body intervenes between the electrodes. A forward portion of the solid electrolyte body is exposed to exhaust gas. The solid electrolyte body is activated through application of heat from the heater. On the basis of electromotive force which is generated between the two electrodes according to a difference in partial pressure of oxygen between exhaust gas and reference gas (the air), the detection element detects oxygen contained in exhaust gas and provides output outward through a pair of lead wires.
A gas sensor having such a structure is required, even when the solid electrolyte body is partially inactive as a result of failure to receive a sufficient amount of heat from the heater, to be free from its influence on output of the detection element. Furthermore, the gas sensor is required, even when an electrode portion sublimates as a result of application of heat from the heater, to exhibit high durability so as to be resistant to consumption of the electrode portion caused by the sublimation. Accordingly, there is known a gas sensor in which the measuring electrode provided on the outer circumferential surface of the solid electrolyte body is formed along the full circumference of the solid electrolyte body at a forward end portion of the detection element and reduces in its circumferential range of formation toward the rear end of the detection element and in which the heater is in contact with the inner circumferential wall of a forward end portion of the solid electrolyte body (refer to, for example, Patent Document 1). According to the invention described in Patent Document 1, a high-temperature region appears at a forward end portion of the solid electrolyte body because of the contact of the heater; however, the range of formation of the electrode reduces with distance from the high-temperature region. Thus, when the detection element provides output outward, influence of an inactive region of the solid electrolyte body can be reduced. At the same time, the range of formation of the measuring electrode is large at a forward end portion of the solid electrolyte body where a high-temperature region appears; therefore, the measuring electrode is resistant to consumption caused by sublimation. Accordingly, the gas sensor has high durability against heat.